A two-dimensional imaging array is shown in a perspective view in FIG. 8. The array includes a two-dimensional array of photodiodes 1, for example, 128.times.128 photodiodes, for generating electrical signals in response to light incident on the respective photodiodes. The electrical signals generated comprise an electrical representation of the distribution of the incident light over a two-dimensional area. That distribution of light, to the extent it is radiated by a source, is referred to here as the image. The photodiode array is mounted on and electrically connected to a silicon substrate 2 containing signal processing circuitry. Each photodiode is in electrical communication with a respective signal processing circuit in the substrate 2 through a columnar body 3, such as a cylindrical volume of indium. This construction is particularly applicable to an infrared light detector in which incident infrared rays 4 strike the photodiode array 1. In that application, the photodiodes may be made of a semiconductor material, such as cadmium mercury telluride (Cd.sub.x Hg.sub.1-x Te), that, unlike silicon, responds to infrared light by generating electrical charges. An infrared-sensitive array like that of FIG. 8 is described in U. S. Pat. No. 4,801,991. Together, the photodiode array 1, the silicon substrate 2 containing signal processing circuitry, and the columnar connectors 3 comprise an imaging array 6.
The general electrical arrangement of the imaging array 6 is shown schematically in FIG. 9 for a 3.times.3 array of photodiodes. The signal processing circuitry in the substrate 2 includes a charge coupled device (CCD) including, in the embodiment of FIG. 9, a horizontal CCD 2a to which three vertical CCDs 2b are connected. The orientation of these CCDs is referred to here as horizontal and vertical because of the orientation of FIG. 9 and other figures. However, no limitation is implied by describing the CCDs as horizontal and vertical. These orientational terms are used only for convenience and refer to two groups of generally orthogonal CCDs in which a plurality of CCDs oriented along one direction are interconnected to a single CCD that is generally orthogonal to the interconnected CCDs. As shown in FIG. 9, each photodiode 1 supplies electrical charges to one of the vertical CCDs 2b through which those charges are transferred to the horizontal CCD 2a. The collected charges are further transferred by the horizontal CCD 2a to an external device through an output circuit element 5, indicated as an amplifier in the figures.
In FIG. 10, the imaging array 6 is interconnected with other components in an imaging apparatus. The imaging array provides electrical imaging signals, each signal representing the intensity of incident light at one of the locations of a photodiode array in a prearranged sequence related to photodiode locations, to a scan converter 7. The scan converter 7 uses the sequential electrical imaging signals to reconstruct a two-dimensional image which is displayed on a television monitor 8. To coordinate the reading out of the sequential electrical imaging signals from the array 6 with the conversion of those signals into a two-dimensional image in the scan converter 7, a timing generator 9 provides timing signals to both the array 6 and the scan converter 7.
Referring to FIGS. 8, 9, and 10, incident light causes the photodiodes 1 to produce electrical charges that are conducted through the connectors 3 to the respective signal processing circuits in the substrate 2. That signal processing circuit stores the electrical charges and eventually transfers them in a timed sequence through the vertical CCDs 2b to the horizontal CCD 2a. The horizontal CCD 2a further transfers the stored electrical charges in a timed sequence through the output circuit 5 to the scan converter 7. The signals are prepared by the scan converter 7 to meet the specifications, e.g., length and number of lines, scanning rate, and so on, of the monitor 8 and are subsequently displayed on the monitor as an image.
The structure of the array 6 shown in FIG. 8 is particularly useful in an infrared detector where the photodiode array is made of a different material from the silicon substrate 2. In the detection of infrared light having wavelengths of about 10 microns, for example, the photodiodes may employ Cd.sub.0.2 Hg.sub.0.8 Te. As understood in the art, the most difficult problem in detecting 10 micron band infrared images is the presence of significant amounts of background radiation. The background radiation is essentially noise that reduces image contrast. The technique commonly employed in other photodetectors to improve signal contrast, i.e., lengthening the time during which the signal charges are collected, is not successful in a photodetector responding to the 10 micron infrared band. Longer charge collection times only increase the quantity of charges generated by the background radiation, resulting in no signal-to-noise improvements.
The effects of the background radiation may be reduced by the charge skimming technique described by Chow et al in IEEE Transactions On Electron Devices, Volume ED-29, Number 1, January 1982, pages 3-13. Here, FIGS. 11(a)-11(c) schematically illustrate signal processing circuitry of the type incorporated in the substrate 2 and employing the charge skimming method. When that charge skimming method is employed, an additional skimming terminal 19, also designated V.sub.SK, is connected to the signal processing circuitry as schematically shown in FIG. 7. The circuitry of FIG. 7 is analogous to that of FIG. 9 but includes an additional connection from each of the signal processing circuits associated with the respective photodiodes 1 to the terminal 19.
Turning to FIG. 11(a), the photodiode 1 generates electrical charge in response to incident light 4. That charge flows into the signal processing circuitry at a contact 20 in the substrate 2. The inflowing charge creates a mirror charge region, i.e., the signal charge, in the substrate 2 indicated by the hatched area opposite the contact 20 in FIG. 11(a). Those signal charges are further transferred from opposite the contact 20 to a potential well where they are stored. Charge transfer to the storage well is controlled by the magnitude of the voltage that is applied to a gate electrode 10. The storage well is created opposite a storage electrode 11 in response to a voltage applied to that electrode. Upon creation of the storage well and sufficient reduction of the barrier between it and the signal charge accumulation opposite the contact 20 by the application of a gate voltage to the gate electrode 10, the signal charges are transferred to and stored in the storage well. This charge transfer operation is illustrated in FIG. 11(a).
As illustrated in FIG. 11(b), a portion of the charge stored in the storage well is transferred to another potential well created opposite a CCD electrode 13. The transfer takes place upon the lowering of the potential barrier between the storage and CCD wells. That barrier is lowered by the application of a skimming voltage to the skimming electrode 12. As shown by the broken line in FIG. 10(b), depending upon the magnitude of the skimming voltage, only an uppermost portion of the charge stored in the storage well has sufficient energy to flow over the lowered barrier and into the CCD well. Thus, the stored charge is "skimmed" in response to the voltage applied to the skimming electrode 12.
After the skimming transfer, the potential barrier between the storage and CCD wells is restored. The remaining charge in the storage well is no longer needed and is drained through a drain electrode 14 of a field effect transistor (FET) by the application of a voltage to the gate electrode 15 of the FET. This drainage of the unneeded stored charge is illustrated in FIG. 10(c). There, the charge stored opposite the CCD electrode 13 is also illustrated.
Through charge skimming, signal charges produced by the respective photodiodes 1 in response to background radiation, i.e., a direct current component, are discarded. The removal of this noise component, which improves the contrast of the image that may be produced, is illustrated in FIGS. 6(a) and 6(b). In FIG. 6(a), the quantity of charge produced by each of a number of picture elements in the array, i.e., the photodiode and signal processing circuitry, is plotted. The white area 16 for each array element represents the charges produced in response to background, i.e., non-image, incident light. The hatched area 17 for each array element represents the charges produced in response to the incident image radiation. The broken line 18 indicates a charge skimming level such that charges below line 18 are discarded through the drain terminal 14 of FIG. 11(c). The charges above line 18 are retained to produce the charge quantity as a function of array element shown in FIG. 6(b). Comparison of FIGS. 6(a) and 6(b) shows that the desired image charges after skimming represent a much larger proportion of the total charge than without the application of the skimming technique. In other words, skimming improves the signal-to-noise ratio.
The illustration of the charge skimming technique shown in FIGS. 6(a) and 6(b) assumes that the same charge skimming level is applied to each of the array elements. That result is conveniently achieved by the circuitry of FIG. 7 where each of the skimming electrodes 12 is electrically connected to the skimming terminal 19. Use of a uniform skimming level in the charge skimming technique provides satisfactory performance when each of the photodiodes in the array has substantially the same light response characteristics. As a result, the prior art teaches that uniformity in the sensitivity of each photodiode in an array is an important goal. However, it is frequently difficult or impossible to achieve uniform photosensitivity among the photodiodes. For example, some semiconductor materials, such as Cd.sub.x Hg.sub.1-x Te used in photodiodes sensitive to infrared light, have characteristics that are notoriously difficult to control, making the achievement of uniform sensitivities among many photodiodes virtually impossible. When the photosensitivities of the photoresponsive elements vary significantly within the array, the advantages of the conventional charge skimming technique with conventional sensors are partially or totally lost.
An example of significant variations in photodiode response within an array is illustrated in FIG. 5(a). There it can be seen that the area 16 corresponding to the background charges varies significantly amongst array elements. In order not to lose signal charges, the skimming level 18 has to be less than the background response of the least sensitive element in the array. As a result, the skimmed charges illustrated in FIG. 5(b) provide little improvement in discarding the background radiation compared to the response of FIG. 5(a).
A proposed solution, teaching away from the prior art and according to one aspect of the present invention, for applying the charge skimming technique to a photoresponsive array in which the light sensors have widely varying sensitivities is the application of different skimming levels to the respective signal processing circuits. In that novel technique, the skimming levels are chosen to compensate for the sensitivities of the associated photodiodes. Application of different skimming levels to different array elements is illustrated in FIGS. 4(a) and 4(b). In FIG. 4(a), the background charge quantity 16 produced by each array element and the corresponding signal charge 17 produced by each element is illustrated as in FIGS. 5(a) and 6(a). Rather than applying a single skimming level 18 to each of the photodiodes, a skimming level 101 that may be different for each array element is applied to the respective photodiodes. The resulting skimmed signal is illustrated in FIG. 4(b). As can be seen by a comparison of FIGS. 4(b) and 5(b), the desired improvement in signal-to-noise ratio, i.e., contrast, achieved in FIG. 6(b) is also achieved in FIG. 4(b).
An imaging array for applying variable magnitude skimming levels directly to respective signal processing circuitry could include a separate skimming electrode terminal for each of the signal processing circuits. In a 128.times.128 photodiode array, however, that arrangement would require more than 16,000 terminals. An imaging array with such a large number of terminals is extremely difficult, if not impossible, to realize practically, particularly in arrays including enough photodiodes to provide a useful image.